An insulated gate bipolar transistor (IGBT) is a switching element controlling a current applied between a collector-electrode and an emitter-electrode by a voltage applied to a gate-electrode. A controllable power of the switching element extends from tens of watts to hundreds of thousands of watts, and a switching frequency thereof also has such a wide range from tens of kilo-hertz to more than a hundred of hertz. This feature is an advantage to widely use the IGBT for a low-power electric appliance for household use such as an air conditioner or a microwave oven and a high-power electric appliance such as a railroad or a rolling-mill driving inverter in steel plants.
The IGBT is generally used in a form of an IGBT module. The IGBT module is a semiconductor device in which an IGBT and a diode connected to each other in inverse-parallel (antiparallel) are incorporated into one package, and an electrode terminal is taken out to the outside, and the IGBT module is mainly used in an inverter device or the like.
FIG. 10 is a diagram showing an inverter using a commonly-used IGBT module. In FIG. 10, reference numerals 100a to 100f denote IGBTs, reference numerals 101a to 101f denote diodes, a reference numeral 110 denotes a motor, and a reference numeral 120 denotes a power supply. As the IGBT module, there are one in which only a pair of IGBT and diode is incorporated in one package, one in which upper arms and lower arms corresponding to one phase are incorporated therein, and one in which upper arms and lower arms corresponding to three phases are incorporated therein, which are selectively used according to usage. For achieving low loss and low cost of the inverter device, low loss and low cost of the IGBT module is required.
Regarding the achievement of low loss, recently, improvement for reducing switching loss of the IGBT is progressed. For example, it is known that, after a collector current is steeply lowered due to turn-off which becomes a main factor of switching loss, a current decreased gently like a tail (hereinafter, called as tail current) is reduced.
Regarding the achievement of low cost, it is known that not a conventional epitaxial grown silicon crystal but an inexpensive floating-zone-grown silicon crystal (FZ crystal) is used as material of IGBT and diode.
As a technique for achieving such low cost and low loss of the IGBT as described above, an IGBT structure of a conventional technique described in Japanese Patent Application Laid-Open Publication No. 2002-314083 (Patent Document 1) is shown in FIG. 11. In the structure, an FZ crystal substrate forms an n-type drift layer 1, a p-type base layer 2 having a doping concentration higher than that of the n-type drift layer 1 is selectively formed on one surface of the FZ crystal substrate, and further an n-type source layer 3 having a doping concentration higher than that of the p-type base layer 2 is selectively formed on a surface portion of the p-type base layer 2. The surface portion of the p-type base layer 2 sandwiched between the n-type drift layer 1 and the n-type source layer 3 is a channel region, a gate insulating layer 4 is disposed on an upper portion of the channel region, and an insulated gate-electrode 5 is further disposed on an upper portion of the gate insulating layer 4, so that an emitter-electrode 6 connected commonly to the p-type base layer 2 and the n-type source layer 3 is formed. Next, an n-type buffer layer 7 is formed by performing ion implantation on the other surface portion of the FZ crystal substrate, a p-type emitter layer 8 is formed by ion-implanting lightly, and a collector-electrode 9 connected to the p-type emitter layer 8 is formed.
In the IGBT, by forming the p-type emitter layer 8 thin introducing ion implantation lightly, injection of holes from the p-type emitter layer 8 can be suppressed to reduce turn-off loss at switching. The IGBT is low in cost because an inexpensive floating-zone-grown crystal (FZ crystal) is used.